Itsumo Baka ni shitekuru Bishoujo-tachi to Zetsuen shitara, Jitsu wa Ore no Koto ga Daisuki Datta you da. - Vol. 2 Ch. 11 - Girls in love are invinci…

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I feel like the author lost his own plot here...
We never saw her with another man. So I'm guessing it's him imagining it in his dream?
She only told him "I've found someone else I like so let's break up" not that he "cheated on her" or whatever he is talking in page 15.
Page 15 is either very badly translated or the author is having trouble remembering his own lore...

Exactly. We specifically saw her thoughts that it was all a lie to get an emotional response out of him. She never cheated. Also on pg 15 Yuuta is saying that SHE said HE was cheating. That is 180 from what was actually said. The only thing that his quotes get right is that she said that she never knew what he was thinking. I doubt it's supposed to be reconning and the HE cheating on HER thing cannot be what he thinks since it mismatches with his dream so it has to be a poor translation.